Unit 4.2 Reflection & Color

In one dimension, reflected waves simply travel back in the direction from which they came. In two dimensions, the situation is a little different. The direction of incident and reflected waves is described by straight-line rays. Incident rays and reflected rays make equal angles with a line perpendicular to the surface, called the normal. The angle between the incident ray and the normal is the angle of incidence. The angle between the reflected ray and the normal is the angle of reflection. Angle of incidence = Angle of reflection The law of reflection states that the angle of incidence and the angle of reflection are equal to each other. The law of reflection applies to both partially reflected and totally reflected waves. If a candle flame is placed in front of a plane (flat) mirror, rays of light from the candle are reflected from the mirror in all directions. Each of the infinite number of rays obeys the law of reflection. The rays diverge (spread apart) from the tip of the flame, and continue diverging from the mirror upon reflection. These divergent rays appear to originate from a point located behind the mirror.

You perceive the candle flame to be located behind the mirror. A virtual image appears to be in a location where light does not really reach. Plane mirrors produce only virtual images. Your eye cannot ordinarily tell the difference between an object and its virtual image. The light enters your eye in exactly the same manner as it would if there really were an object where you see the image. The image is the same distance behind the mirror as the object is in front of it. The image and object are the same size. The law of reflection holds for curved mirrors. However, the sizes and distances of object and image are no longer equal. The virtual image formed by a convex mirror (a mirror that curves outward) is smaller and closer to the mirror than the object is. When an object is close to a concave mirror (a mirror that curves inward), the virtual image is larger and farther away than the object is.

Ordinary paper has a rough surface when viewed with a microscope. Diffuse reflection is the reflection of light from a rough surface. Each ray obeys the law of reflection. The many different angles that incident light rays encounter at the surface cause reflection in many directions. Visible light that reflects from a sheet of paper is diffusely reflected. Rays of light incident on paper encounter millions of tiny flat surfaces facing in all directions, so they are reflected in all directions. Diffuse reflection allows us to read the page from any direction or position. We see most of the things around us by diffuse reflection. Diffuse reflection allows us to see most things around us. Light is diffusely reflected from paper in many directions. Light incident on a smooth mirror is only reflected in one direction. Ordinary paper has a rough surface when viewed with a microscope.

Sunlight is an example of what is called white light Sunlight is an example of what is called white light. White light is a combination of all the colors. When sunlight passes through a prism, it separates into a spectrum of all the colors of the rainbow. Black is similarly not a color, but is the absence of light. Objects appear black when they absorb light of all visible frequencies. The colors of most objects around you are due to the way the objects reflect light. The color of an opaque object is the color of the light it reflects. This square reflects all the colors illuminating it. In sunlight, it is white. When illuminated with blue light, it is blue. b.This square absorbs all the colors illuminating it. In sunlight it is warmer than the white square. When white light falls on a flower, light of some frequencies is absorbed by the cells in the flower and some light is reflected. Cells that contain chlorophyll absorb light of most frequencies and reflect the green part, so they appear green. The petals of a red rose, on the other hand, reflect primarily red light.

Petals of most yellow flowers, such as daffodils, reflect red and green as well as yellow. Yellow daffodils reflect light of a broad band of frequencies. The reflected colors of most objects are not pure single-frequency colors, but a spread of frequencies. So something yellow, for example, may simply be a mixture of the colors red and green together. Light of all the visible frequencies mixed together produces white. White also results from the combination of only red, green, and blue light. Red and green light alone overlap to form yellow. Red and blue light alone produce magenta. Green and blue light alone produce cyan. This amazing phenomenon is due to the way the human eye works. The eye has three types of color sensitive cells called cones. One type is sensitive to red light, a second type is sensitive to green light and the third type to blue light. They send signals to the brain which interprets them as all the colors you see.

When two of the three additive primary colors are combined: red + green = yellow red + blue = magenta blue + green = cyan When we add in the third color, we get white: yellow + blue = white magenta + green = white cyan + red = white When two colors are added together to produce white, they are called complementary colors. Yellow and blue are complementary because yellow is the combination of red and green. Red, green, and blue light together appear white. By similar reasoning we see that magenta and green are complementary colors, as are cyan and red. Begin with white light and subtract some color from it. The resulting color appears to be the complement of the one subtracted. Some of the light incident upon an object is absorbed. The part that is absorbed is in effect subtracted from the incident light. For example, if white light falls on a pigment that absorbs red light, the light reflected appears cyan. Subtract a color from white light and you have the complementary color. When white light passes through all three transparencies, light of all frequencies is blocked (subtracted) and we have black. Remove blue & red light Remove blue light Remove red light Remove blue & green light Remove green light Remove green & red light

Red and green paint do not combine to form yellow as red and green light do. The mixing of paints and dyes is an entirely different process from the mixing of colored light. Paints and dyes contain particles of pigment that produce colors by absorbing light of certain frequencies and reflecting others. When paints or dyes are mixed, the mixture absorbs all the frequencies each paint or dye absorbs. Blue paint, for example, reflects blue light and it absorbs red and green. Yellow paint reflects mostly yellow light, but also red and green; it absorbs blue light. Mixing colored light is called color mixing by addition. When you cast lights on a stage, you use the rules of color addition, but when you mix paint, you use the rules of color subtraction. The three colors most useful in color mixing by subtraction are magenta, yellow and cyan. These are the colors used in printing illustrations in full color. Color printers use four differently colored inks (magenta, yellow, cyan, and black). Each color of ink comes from a different plate, which transfers the ink to the paper. The ink deposits are regulated on different parts of the plate by tiny dots. The overlapping dots of three colors plus black give the appearance of many colors.